Q-switching is a method used in lasers to produce short intense pulses of light. In a laser the active gain medium is enclosed in a resonator, such as comprising a pair of mirrors, to permit the laser beam to transit repeatedly through the gain medium and build up in strength. The Q of the cavity is a measure of the cavity loss. A high Q means that the loss is low and light may reflect many times between the mirrors, while a portion also transmits out through one mirror with each round-trip. A low Q means that the loss is high. If the gain medium is pumped while the cavity Q is low, energy is stored in the gain medium without being extracted, since the circulating light intensity is low. Q-switching refers to permitting the energy to build in the gain medium (high population inversion) in such a low-Q state, and then rapidly switching the cavity to a high-Q state. This has the effect of rapidly building up a pulse of light that efficiently extracts the stored energy and outputs the light as a short intense light pulse.
Q-switching is frequently done using active means, in particular using electro-optic or acousto-optic devices to rapidly switch between low and high Q states. This works well but means that actively driven and frequently expensive means must be added to the laser cavity. For short pulse generation active approaches can also be problematic because the temporal width of a laser pulse is proportional to the length of the laser cavity and adding bulky (such as several cm long) elements in the cavity can force the pulses to be longer than desired.
An alternative method is to passively Q-switch the laser using a saturable absorber (SA), such as cobalt spinel. These materials normally absorb light. However, when the intensity incident on them reaches a certain threshold (bleaching point) the material can no longer absorb more light and becomes transparent. Insertion of such a device in a laser cavity therefore permits one to operate the laser at first in a low-Q state when the saturable absorber is absorbing, and then in a high-Q state once the saturable absorber reaches the bleaching point. If the loss of the SA is chosen properly light intensity builds up slowly in the cavity until the intensity bleaches the SA. At that point the cavity Q is switched to a high state and the cavity produces a short pulse. In the case where the gain medium is pumped continuously a cycle is produced wherein the stored energy builds up in the gain medium, the Q-switching action extracts the energy into a pulse, the SA returns to the absorbing state within a characteristic time, the energy starts to build up again and another pulse is produced. As a result these lasers produce a train of pulses whose pulse repetition frequency is dependent on the laser design, and in particular on the characteristics of the saturable absorber. If the SA is made too absorbing (too optically thick) the laser light never builds up to a high enough intensity to bleach and Q-switching never takes place. If the material is made insufficiently absorbing (too optically thin) the low-Q state is not realized and Q-switching also doesn't take place. Between these regimes there is a region where the thickness of the material can be selected as a compromise to produce pulses at a specific pulse repetition frequency (PRF).
A limitation with passive Q-switching is evident from this discussion. For a given laser configuration, once the thickness of material has been chosen, the PRF of the laser is fixed and can only be altered by replacing the Q-switch with a different thickness material. This is inconvenient if one desires to vary the pulse rate. In addition, changes in the pumping of the gain medium (including aging effects) over time may alter the time it takes to build up enough intensity to bleach the Q-switch. This can have the inadvertent effect of the PRF changing with time.
In U.S. Pat. No. 6,466,593 a device is described to circumvent this limitation. The device consists of two wedges of saturable absorbers that are moved relative to one another in such a manner that the total optical thickness of the SA can be changed. In principle this device works, but it has several practical limitations. First, the device requires two wedges to move in opposite directions along the same axis. This is very inconvenient to implement. Second, because both wedges must move, there has to be a space between the wedges to prevent wear. Third, since the thickness of the material changes as the wedges are moved, the length of the laser cavity also changes. For short laser cavities this can become very problematic. Not only does this alter the laser PRF, but it can also lead to changes in the diameter of the resulting laser beam, which is frequently highly undesirable.
A need, therefore, remains in the art for methods and apparatus for varying pulse repetition frequency in passively Q-switched lasers which overcome the disadvantages of previous inventions.